1. Field of the Invention
The present invention relates to an optical system, and particularly to an optical system preferably usable in a variety of apparatuses, such as a digital camera, a video camera, a film camera, a binocular, a projector, a telescope, a microscope, and a copying machine.
2. Related Background Art
Conventionally, a method of combining lenses made of glass materials having different dispersions has been employed to correct chromatic aberrations of an optical system. In contrast to such a method of reducing chromatic aberration by the combination of glass materials, known is a method of reducing chromatic aberration by using a diffractive optical element (DOE) having a function of diffraction in an optical system (see Japanese Patent Application Laid-Open No. H04-213421 (corresponding to U.S. Pat. No. 5,044,706), and Japanese Patent Application Laid-Open No. H06-324262 (corresponding to U.S. Pat. No. 5,790,321).
For example, the following optical system is known as an optical system using a diffractive optical element. The optical system is a zoom lens including a first lens unit having positive refractive power, a second lens unit having negative refractive power, and following lens unit(s) in order from an object side to an image side, and the first lens unit is provided with a diffractive optical element (see Japanese Patent Application Laid-Open No. H11-133305 (corresponding to U.S. Pat. No. 5,978,153).
Further, in a diffractive optical element including a diffraction grating formed of a material having a low ultraviolet-radiation resisting characteristic, known is a diffractive optical element having a light blocking means for blocking or screening ultraviolet radiation, such as an ultraviolet-radiation cut filter, disposed on a incident light receiving side (see Japanese Patent Application Laid-Open No. 2001-264526 (corresponding to U.S. Pat. No. 6,522,464).
Further, known is a lens system in which a protective glass is disposed on a most object side of a telescopic lens, and a coating (ultraviolet-radiation blocking means) for blocking ultraviolet radiation is formed on a lens surface of the protective glass.
Generally, a small-sized total structure of an optical system and an enhanced optical performance are required for an optical system such as a shooting lens.
Furthermore, an optical apparatus (an image pick-up apparatus) to be used in a variety of environments, such as outdoors and indoors, must satisfy a sufficient environmental durability. Therefore, if a diffractive optical element is made of a material having a low ultraviolet-radiation resisting characteristic, ultraviolet-radiation blocking means for protecting the diffractive optical element from ultraviolet radiation and the like must be disposed at an appropriate location in an optical system. Unless such a blocking means is disposed, optical performance of the diffractive optical element is likely to deteriorate, leading to a decrease in optical performance of the entire optical system.